Is Resulting Binary from Changing Proto Definition Source (From Nested to Non-Nested) for Fields initialization Backward-Compatible?

[Felik]

Hello,

I came to understand that the binary serialization only concern of field identifiying number (hence renaming field and message is backward compatible).

I would like to confirm my understanding if following case is also backward compatible (I could not find any resource that discuss about this, or I may have used the incorrect search term for this).

Let's say previously I have defined a nested proto definition as follow:

message OuterProto {

   message InnerProto {

      optional bool field_bool = 1;

      optional int64 field_int = 2;

   }

   optional InnerProto field_composed = 1;

}

Now, I would like to "un-nest" the InnerProto definition by redefining another duplicate proto in top level as follow:

// same exact field numbering & type

message NewInnerAsTopLevelProto {

      optional bool field_bool = 1;

      optional int64 field_int = 2;

}

Would following modification to message OuterProto be backward-compatible ?

message OuterProto {

   message InnerProto {    // unused

      optional bool field_bool = 1;

      optional int64 field_int = 2;

   }

   optional NewInnerAsTopLevelProto field_composed = 1;

}

Any input & insight would be much appreciated. Thank you!

[Marc Gravell]

As long as you aren't using `Any`: then at the binary payload data it won't be visible. Any existing code that uses the generated types will need to be updated, obviously.

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[Timo]

google.protobuf.Any stores the qualified name of the packed message as a string. It is only a breaking change if InnerProto was stored in Any, not if OuterProto was stored.